As indicated in the article entitled "Clams--A Growing Threat To Inplant Water Systems" Plant Engineering, June, 1979, page 165 et seq., Corbicula ssp, the so called asiatic clam, has become and currently is a major problem in industrial plants and utility systems which utilize large quantities of water particularly for cooling water and more particularly in once-through cooling water systems.
For further information see also Mattice, J. S.,1979. "Interactions of Corbicula ssp, with power plants," pages 119-138 and Goss, L. B. et al, 1979. "Control Studies on Corbicula for steam electric generating plants," pages 139-151. In J. C. Britton (ed) Proceedings, First International Corbicula Symposium, Texas Christian University Research Foundation, Fort Worth, Tex., 313 pages.
Cooling systems which use cooling water on a one-time basis before discharging it directly to waste are termed oncethrough cooling systems. Since even small cooling systems operating on a once-through basis use relatively large amounts of cooling water, these systems are generally employed only where water at a suitably low temperature is readily available in large volumes and at low costs.
The usual source of once-through cooling water is from wells, rivers and lakes where the only cost involved is that of pumping. Generally, the only external treatment applied to once-through waters taken from rivers and lakes is screening to remove objects which are large enough that they could damage pumps and heat exchanger equipment.
Asiatic clams (Corbicula spp) were first discovered in the United States in 1938 in the Columbia River in Washington. Since then the range of this exotic species has widely expanded and presently it is found in at least thirty-three states. The spread of this species has been of special interest because of problems it has caused in the water supply systems in electric power plants, industrial plants and municipalities.
As indicated in the above identified article from Plant Engineering:
"Infestation is usually limited to that part of an industrial plant or operation that uses fresh, untreated lake or river water. The majority of problems result from plugging by clam shells. Adult shell-bearing clams may be drawn into a plant's water system and carried to a condenser, where they can plug the tubes. In most cases, the organisms enter the system in the larval stage and develop to maturity in areas of low water velocity. When the clams die, their shells may be carried by the water to areas susceptible to plugging. These can include cooling systems for compressors, condensers, blast furnaces, and other processes. PA1 Live clams have also been found in the media of raw-water pressure filters, upstream of softeners, and in solids depositions in water basins. Normally, cooling-tower water systems are free of the organisms, because such systems have relatively high-temperature water and receive biocide treatment. PA1 One-year-old clams are capable of plugging valves and nozzles. Two-year-old clams can cause mechanical damage to impellers and other moving parts of water-distribution systems. At six years, the clam can damage tires of construction vehicles. As in all other clams, growth is rapid in early years and then tapers off." PA1 Primary considerations in selecting a plant control procedure for Corbicula larvae are the compatibility of the method with the water system, the ability to meet effluent and stream water-quality standards if a chemical control agent is used, and economics. Because the clams cannot live in temperatures above 78.degree. F. (26.degree. C.), heating can provide effective control. However, this method is not applicable to all water systems. In some systems, the water volume that must be heated is too large or the system's design will not allow the heated water to reach all areas suitable for clam propagation. PA1 Control with chlorine is somewhat limited because it is difficult to maintain the necessary chlorine concentrations in static water lines. PA1 Mechanical devices, such as traps, can be used to prevent mature clams from entering water systems and for removing clams or shells from systems. Screens and traps may not be effective in preventing clam larvae infestation, but they often are used as an integral part of a general biological control program. PA1 The organism must be controlled in the freefloating or juvenile stages to prevent the development of problem-causing mature, shell-bearing clams.
The clams are very tolerant of many chemicals and often occur in great abundance. They have accumulated to depths of two meters in the Delta-Mendota Canal in California and have caused reductions in water flow. Some industrial plants have had difficulty obtaining fire insurance after inspectors found the fire protection systems plugged with Corbicula shells. Pump impellers have been damaged by shells in some industrial plants. The number of power plants which have experienced problems with this species has increased during the past several years. Problems in fossil-fueled power plants most often relate to pluggage of condenser tubes, surface water heat exchangers, and blockage of fire protection systems. In addition to these problems nuclear power plants may have other problems associated with the shutdown service water, and emergency reactor cooling systems. In April of 1981 the Nuclear Regulatory Commission issued the Inspection and Enforcement Bulletin #81-03 concerning the flow blockage of cooling water to safety system components by Corbicula.
An examination of the biology of the Corbicula provides some explanation of how this organism is able to cause such serious problems in water supply systems. The Asiatic clams have a strong, thick shell which is almost spherical in shape. They normally obtain a shell length of 20-40 mm (depending on the environment) but become sexually mature at only 10 mm. Corbicula are hermaphroditic so every mature clam produces eggs which are incubated in the gills and released as a larval form called a veliger. They may spawn continually at water temperature above 16.degree. C. (60.degree. F.). Published information indicates that during the spring and fall spawning peaks each clam may release as many as 588 veligers per day. When one considers that adult Corbicula can attain a density of 20,000/m.sup.2, it follows that several million veligers could be released from each square meter per day. The diameter of the veliger when it is released is about 200 microns (0.008 inches) and it is a weak swimmer usually found near the bottom of the water column. The small size and tremendous number of veligers explains how they can so successfully gain entrance to untreated water systems. If conditions are favorable, these veligers can develop into clams with a shell length of 10-15 mm (depending on the environment) within a year and create problems in water lines. The adult clams are very tolerant of many chemicals and their thick shells can cause problems even after the clams have died.
The differences in size and chemical tolerances of the various life stages of Asiatic clams have complicated attempts to control them. Controls are also complicated by the different types of problems that clams have caused in various water systems. Because of these complexities, it appears that a single control measure has not previously been effective for all life stages or all water systems and control methods have had to be specific in their application. A variety of control measures have been studied for use in power plants and one of the most effective controls is one which is designed for static water systems such as fire protection systems, and consists of a 0.8 mm (1/32 inch) filter screen and chlorine injection. Chlorine at 0.5 ppm for 72 hours will result in 100% mortality of the veligers. This method offers probably the best control but presents environmental and regulatory problems. Regulations limiting the discharge of chlorine would prohibit its use, although variances are sometimes granted.
According to the Plant Engineering article: